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Jun 24, 2021 - 4:40:20 AM
264 posts since 2/16/2017

Hi,

A couple of years ago I made my first 10" 5 string travel banjo, it's played great ever since and I'm currently making another.

The scale length is 588mm with 19 frets.

For the new one I thought I would use Stewmac's fret calculator and create a new template for the frets. I created the the template each time measuring from the nut and positioning my guides exactly where Stewmac say they should be. So, in theory, and as it's been set up by computer to the .001 mm it should be quite an accurate template.

Here's the problem, it's no where near my old banjo's fret positions. By the time you get to the 9th fret it's half a fret different?

From memory and not knowing anything about scale lengths etc when I first attempted this, I think I may have found a template online, possible 22 or 24 frets copied it and just removed the frets after the 19th. As I'm still a beginner i've not ventured past the 12th fret in my playing but even the 12th fret looks to be in a different position.

What I don't understand is how my old banjo sounds ok if it's this far out? And if I create the new one using the calculations from Stewmac will this be out? In my head I can't see how they would both work.

Any ideas?


 

Edited by - hardleydavidson on 06/24/2021 04:43:47

Jun 24, 2021 - 5:00:10 AM
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10966 posts since 4/23/2004

The first thing I would do is compare the distance from the nut to the twelfth fret. Do not measure to the bridge (as it is movable).

Doubling the above gives your scale. Conversely: 588 ÷ 2 = 294. If you don't get that, something is wrong.

I just did the Stewmac calc and got 294mm for the 12th fret. Check your original banjo's 12th fret.

Copying is fraught with problems, direct measurement is exact.

Edited by - trapdoor2 on 06/24/2021 05:06:09

Jun 24, 2021 - 5:07:45 AM
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150 posts since 7/14/2017

I'd guess you measured nut to bridge and used that for scale length. Nominal scale length is nut-12th x 2. Actual string length will be around 2mm longer to compensate for stretching the strings while fretting.

Measure the nominal length and I suspect the calculations will match.

Jun 24, 2021 - 5:21:26 AM
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264 posts since 2/16/2017

Oh how right you are, thanks Marc,

So when I was calculating my scale length I went from my original design on screen as it gave me all the measurements instantly, I've just physically measured the old banjo and it's 285 from the nut to the 12th, somewhere along the line it lost 9mm in it's creation. (or quite possibly I'm using the wrong design file). So much for computer aided design :D

Thought I was going mad!!

Jun 24, 2021 - 6:19:29 AM
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13700 posts since 6/29/2005
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Here's a spreadsheet for a 23.15" (588mm) scale taken out to three decimal places using 23.15/17.817154 as the starting point in 2B.  17.817154 is the standard fret spacing formula—If you divide any scale length by the constant 17.817154, you will get the distance from the front edge of the nut to the first fret, and so on and so forth,

Sorry it's in inches, but this may help—use the right hand-column—cumulative fret spacings from the first fret down.

I don't know how to attach the actual spreadsheet, or I would.

Jun 24, 2021 - 7:10:41 AM
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1257 posts since 8/7/2017

If you tune to Just Intonation frequencies, rather than the piano standard of Equal Temperament (ET) frequencies (also used by regular electronic tuning aids), you will find it takes different fret spacing for each string. And the spacing is dependent on the key you tune to. That is, change key, and you need to change fret spacing. This has been explored, in detail, by a Turkish guitar luthier&music professor; he will build you a fretboard with moveable frets (for guitar). I discuss Just Intonation, at length, in the archived thread here:
banjohangout.org/archive/341664

Perhaps your banjo's original fret spacing sounds fine to your ears, whether they are in the perfectly theoretical positions or not. Or you have grown used to the pitches for each original fret. This tolerance for fret spacing errors can vary day to day, by the way, if your ears/brain have a variety of "perfect pitch". Not everyone can hear the difference between Equal Temperament (which is what StewMac template is set up for), and Just Intonation. I will mention that Earl Scruggs could hear the difference on the 3rd string, and had a banjo where he moved one of the 3rd strings' frets. The "compensated" bridges available do this, in a more general way, allowing ET frets to sound better (to me and others with pitch sensitivity, anyway).

If you are making banjos for sale, then I'd stick to the ET fret spacing since it's what most everyone uses and expects. If you are making another banjo for yourself, then let your ears be the guide :-)

Hope this helps.

Jun 24, 2021 - 8:19:12 AM

264 posts since 2/16/2017

Thanks Guys,

Some extremely helpful resources and knowledge there, fretting is the one task I fret (pun intended) about as it's so easy to get it wrong. With this build I've decided to cut the slots then inlay as it's easier to inlay without frets in the way, then fret and bind. Hopefully all will go to plan.

I've decided to stick with the Stewmac and Ken's calculations (I've managed to replicate the spreadsheet using the 17.817154 formula), the slightly longer neck looks much better and I have enough spare wood past the 19th to get the bridge position where I want it without a gap between the neck and the tension hoop.

Thanks chaps!!! :-)

Jun 24, 2021 - 9:43:07 AM
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8722 posts since 8/28/2013

Let's not complcate this. Looking closely at the photo, I can easily see that it's not simply one fret that's off. The difference between the template and the actual banjo starts at the very first fret and gradually grows. Therefore, it appears to me that both are probably reasonably accurate, but the Stew-Mac template was calculated for a longer scale.

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